# std::weak_order

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Defined in header `<compare>` |  |
inline namespace /* unspecified */ { inline constexpr /* unspecified */ weak_order = /* unspecified */; } |  | (since C++20)
Call signature |  |
template< class T, class U > requires /* see below */ constexpr std::weak_ordering weak_order(T&& t, U&& u) noexcept(/* see below */); |  |

Compares two values using 3-way comparison and produces a result of type `std::weak_ordering`.

Let t and u be expressions and `T` and `U` denote decltype((t)) and decltype((u)) respectively, std::weak_order(t, u) is expression-equivalent to:

- If std::is_same_v<std::decay_t<T>, std::decay_t<U>> is true:
  - std::weak_ordering(weak_order(t, u)), if it is a well-formed expression with overload resolution performed in a context that does not include a declaration of `std::weak_order`,
  - otherwise, if `T` is a floating-point type:
    - if std::numeric_limits<T>::is_iec559 is true, performs the weak ordering comparison of floating-point values (see below) and returns that result as a value of type `std::weak_ordering`,
    - otherwise, yields a value of type `std::weak_ordering` that is consistent with the ordering observed by `T`'s comparison operators,
  - otherwise, std::weak_ordering(std::compare_three_way()(t, u)), if it is well-formed,
  - otherwise, std::weak_ordering(std::strong_order(t, u)), if it is well-formed.
- In all other cases, the expression is ill-formed, which can result in substitution failure when it appears in the immediate context of a template instantiation.

### Customization point objects

The name `std::weak_order` denotes a customization point object, which is a const function object of a literal `semiregular` class type. For exposition purposes, the cv-unqualified version of its type is denoted as `__weak_order_fn`.

All instances of `__weak_order_fn` are equal. The effects of invoking different instances of type `__weak_order_fn` on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, `std::weak_order` can be copied freely and its copies can be used interchangeably.

Given a set of types `Args...`, if std::declval<Args>()... meet the requirements for arguments to `std::weak_order` above, `__weak_order_fn` models

- std::invocable<__weak_order_fn, Args...>,
- std::invocable<const __weak_order_fn, Args...>,
- std::invocable<__weak_order_fn&, Args...>, and
- std::invocable<const __weak_order_fn&, Args...>.

Otherwise, no function call operator of `__weak_order_fn` participates in overload resolution.

#### Strict weak order of IEEE floating-point types

Let x and y be values of same IEEE floating-point type, and weak_order_less(x, y) be the boolean result indicating if x precedes y in the strict weak order defined by the C++ standard.

- If neither x nor y is NaN, then weak_order_less(x, y) == true if and only if x < y, i.e. all representations of equal floating-point value are equivalent;
- If x is negative NaN and y is not negative NaN, then weak_order_less(x, y) == true;
- If x is not positive NaN and y is positive NaN, then weak_order_less(x, y) == true;
- If both x and y are NaNs with the same sign, then (weak_order_less(x, y) || weak_order_less(y, x)) == false, i.e. all NaNs with the same sign are equivalent.

### Example

| This section is incomplete Reason: no example

### See also

weak_ordering (C++20) | the result type of 3-way comparison that supports all 6 operators and is not substitutable (class)
strong_order (C++20) | performs 3-way comparison and produces a result of type `std::strong_ordering` (customization point object)
partial_order (C++20) | performs 3-way comparison and produces a result of type `std::partial_ordering` (customization point object)
compare_weak_order_fallback (C++20) | performs 3-way comparison and produces a result of type `std::weak_ordering`, even if operator<=> is unavailable (customization point object)

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